Dry
lubricants can give design engineers and manufacturers a solution to the
familiar design and production challenge of stacked tolerances. Tolerances
refer to the permissible limit or limits of variation in a physical dimension,
and are specified by the design engineer to allow reasonable leeway for
imperfections and inherent variability - without compromising performance of
the finished assembly or process.
This
challenge of stacked tolerances becomes an issue for design engineers when
working with mechanical assemblies that have multiple components and moving
parts that work in unison. One solution that could be used to address stacked
tolerances is to design everything with tighter tolerances, thus ensuring
higher levels of precision. This option is not usually the most appropriate for
the design engineer and manufacturer because of the high cost associated with
it. This is especially true if the finished assembly has a limited service life
expectancy. Higher precision equals higher cost because more frequent
inspections and maintenance of the tooling and machines during manufacturing
are necessary to obtain high levels of precision.
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A
more cost-effective solution to address stacked tolerances is the use of a dry
lubricant as a surface treatment. Dry lubricants using PTFE technology provide
a thin, smooth coating over the finished device or mechanical assembly to
reduce friction caused by stacked tolerances. This ensures device functionality
and performance; in fact, dry lubricants can reduce the force needed to actuate
a device or assembly by 25 to 30 percent.
A
common example of a portable medical device found exclusively in the operating
room that benefits from dry lubricant coating is the surgical stapler. A
stapler is essentially a mechanical assembly that requires trigger activation
to actuate the stapling action. The motion of springs and metal stamped parts
sliding against each other is a process that requires the engineer to design
with precision and allotted tolerances. In high-volume production, the use of
dry lubricant can keep those precision costs down while ensuring device
performance.
Manufacturing
volumes and surface geometries influence what application method to choose for
dry lubricant use. The various methods of application include: dipping, wiping
or brushing, air spraying, air-less spraying, aerosol sprays, drying, and
melt-coating for improved adhesion and durability. The most common method used
is dip-coating because it offers the most consistency from part to part.
Dry
Lubricant Selection Issues
Dry
lubricant fluids are non-flammable and have a high degree of materials
compatibility, permitting use on nearly all types of metals and most plastics
including polycarbonates. Despite their wide applicability, when choosing a dry
lubricant, design engineers must take many variables into consideration.
Consistency and quality of the coating is an extremely important variable
because it can affect how well the lubricant will perform. To address this,
design engineers must pay attention to the calibration of the fluid. In other
words, the ratio of PTFE particles to carrier fluid should remain at consistent
levels throughout the coating process. This can also be impacted by the
evaporation of the carrier fluid.
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Another
challenge to maintaining calibration is the "hang time" of the PTFE particles
within the carrier fluid. It is common for the PTFE particles to sink in the
carrier fluid to the bottom of the vessel, thus affecting calibration and
consistency of the coating. When the part or device is dipped into the fluid,
it may end up with a streaky and inconsistent coating. If manufacturers work to
constantly agitate the fluid, this can help to maintain calibration. However,
constant agitation requires a lot of extra work on the production floor and
also can promote evaporation of the carrier fluid. By choosing a dry lubricant
vendor that provides a ready-to-use, pre-mixed and calibrated formula that has
a high "hang time" and maintains the ratio of carrier fluid to PTFE particles,
design engineers can be assured that they will achieve a more consistent
coating and smoother end-use actuation.
Another
consideration for design engineers when choosing a dry lubricant are the
environmental and safety qualities of the coating. Some vendors offer products
that have better environmental properties, meeting strict Environmental
Protection Agency (EPA) regulations. For example, the
MicroCare DuraGlide has an EPA
designated Volatile Organic Compound (VOC) exempt status and also has an Ozone
Depletion Potential (ODP) of zero.
Jay Tourigny is vice president of operations
at MicroCare Medical. Click here
for more information.
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